James Leighton trained as a horticulturalist at the Royal Botanical Gardens at Kew, near London, where he worked from 1878 to 1880. Early the next year he came to the Cape Colony and settled at King William's Town for the rest of his life. In 1882 he was appointed curator of the King William's Town Botanic Garden, succeeding James Johnson who had been in charge for only a year. Leighton soon started some experiments with imported plants and by 1884 was establishing a collection of economic and medicinal plants such as chicory, varieties of tobacco, sunflower, foxglove, opium poppy, rape, flax, and tea. He resigned his position about the beginning of 1889 and no successor was appointed until Thomas R. Sim* took charge of the gardens in 1892. After his resignation he developed a private nursery which came to be known throughout South Africa. He was elected a Fellow of the Royal Horticultural Society.
Leighton was an early member of the King William's Town Naturalists' Society (founded in July 1884). In 1886 he read a paper on "Some injurious insects" before the society and in November that year was elected (or re-elected) as a member of its management committee. He again (or still) served on the committee in 1897 and that year was a member also of the executive committee of the society's King William's Town Public Museum (later the Kaffrarian Museum). In 1908 he became a member of the South African Association for the Advancement of Science and by 1918 had been elected a member of its council. In 1917 two papers by him were published in the Report of the South African Association for the Advancement of Science (Vol. 14): "Some suitable materials for paper-making" (pp. 287-289), and "Notes on fibre produced from some of the most useful indigenous and exotic plants in the Cape Province" (pp. 443-445).
Leighton served on the Town Council of King William's Town from 1898 to 1922, and as mayor from 1910 to 1911. He was the father of botanist Francis M. Leighton